Embodiments presented herein relate generally to curing techniques for composite materials, and more particularly, to curing techniques for use in manufacturing and repairing damage to structural composite products, such as wind turbine rotor blade laminates.
Wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. With this growing interest, considerable efforts have been made to develop manufacturing and repair techniques for wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted to a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in length) and generally have an average wind turbine rotor blade size of 24 meters to 47 meters in length (80-155 feet). In addition, the wind turbines are typically mounted on towers that are at least 60 meters in height. Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators. Wind turbine rotor blade designs have become increasingly complex to maximize aerodynamic properties and to withstand a variety of environments and conditions.
A wind turbine cannot generate electricity without the wind turbine rotor blades. Generally, wind turbine rotor blades are fabricated from composite fiber materials and matrices, composed of multiple layers or plies. In many instances, if certain material failures occur in the wind turbine rotor blade, the wind turbine may be taken off-line and the wind turbine rotor blade must be replaced or repaired. The costs and time associated with transportation of replacement blades and the installation of the replacement blades is very high. Current methods used to repair wind turbine rotor blades are time and labor intensive and require special repair methods and facilities, as such, fabrication and repair of wind turbine rotor blades is difficult and expensive.
One conventional approach utilized in both manufacture and repair of wind turbine blades involves curing multiple layers, wherein all of the layers are cured together at the same time in an autoclave or oven. In other words, the process involves applying the layers one over the other, and then subsequently curing all of the layers. The adhesion between layers is generally good but other disadvantages sometimes make this approach impractical. For instance, in manufacturing or repairing the root section of a wind turbine blades, sagging and dimensional distortion and fiber wrinkling during compaction may occur during the curing cycle. Also, excessive reaction exotherm from thick parts may cause problems. Another approach for curing these multiple layers involves sequentially applying and completely curing layers one after another using a single curing mechanism. In other words, a first composite layer is cured completely before laying down a second composite layer. The second composite layer is then cured completely before laying down a third composite layer. The process adds additional composite layers in the same sequential manner using the single curing mechanism. Unfortunately, this fabrication technique creates relatively weak secondary adhesive bonds between the composite layers. These secondary adhesive bonds result in undesirably low interlaminar strength.
In alternate situations, in-field repair, including up-tower repair is feasible. In these instances, a thermal blanket heating process may be utilized, requiring a green cure and an extended long post-curing process in order to achieve the desired physical and mechanical properties. The up-tower time required for a long thermal curing process contributes significant man-hour cost for wind blade repair processes. UV curing may provide faster laminate curing as an efficient in-field repair process, however, UV curing processes alone have been limited to thin laminate curing only. It is widely known that UV curing processes alone cannot achieve high glass transition, and full curing of thick composite laminates.
Hence, an improved technique needs to be developed to address the aforementioned manufacture and repair process issues.